Chapter 2. Non-Metallic Structure-Fabric Covering

9/8/98 AC 43.13.1B CHAPTER 2. FABRIC COVERING

SECTION 1. PRACTICES AND PRECAUTIONS

2-.1 GENERAL. Cotton and Irish linen 2-.4 AIRCRAFT FABRIC-SYNTHETIC. fabrics were the airframe coverings of choice from WWI through the 1950’s. However, .a STC-Approved Covering Materials. increases in cost and the short lifespan of There is a wide selection of STC-approved natural fabrics became the driving factors covering materials available which utilize which resulted in almost 100 percent synthetic fabric falling within the generic class replacement of original airframe fabrics by “Polyester” and may vary in characteristics. man-made, STC-approved, polyester, and Difference in the fabric may be denier, glass filament fabric. tenacity, thread count, weight, shrink, tension, and weave style. 2-.2 PROBLEM AREAS. .b Polyester Filaments. Polyester .a Deterioration. Polyester fabric Filaments are manufactured by polymerization deteriorates only by exposure to ultraviolet of various select acids and alcohols, then radiation as used in an aircraft covering extruding the resulting molten polymers environment. When coatings completely through spinnerets to form filaments. The protect the fabric its service life is infinite. filaments are heat stretched to reduce to the Therefore, it is very important to thoroughly desired denier or size. It is the heat stretching protect the structure from deterioration before that imparts a memory in the filaments causing covering and provide adequate inspection them to try and return to their original shorter access to all areas of fabric-covered length when reheated at a controlled components to allow inspection for corrosion, temperature. Overheating will cancel the wood rot, and mice infestation. Multiple drain memory and melt the filaments. holes in the lower ends of all fabric-covered sections also provide needed ventilation to .c Covering Procedures. Coating types, remove condensation. covering accessories, and covering procedures also may vary; therefore, the covering .b Tension. Polyester fabric obtains procedures given in the pertinent manuals maximum tension on an airframe at 350 ºF, must be followed to comply with the STC. and will not be excessive on aircraft originally The FAA STC-approved installation takes covered with natural fabric and 12 coats of precedence over instructions in this advisory Nitrate or Butyrate Dope. However, dope circular. applied over full heat-tauted fabric can develop excess tension after aging and damage .d Installation. Initial installation of light aircraft structures. Coatings other than polyester fabric is similar to natural fabric. dope will not increase fabric tension after The fabric is installed with as little slack as aging. The heat-tauting instructions given in possible, considering fittings and other the manual of each STC-approved covering protrusions. It may be sewn into an envelope, process should be followed. installed as a blanket, or installed by cementing to the airframe with a fabric 2-.3 cement. Each STC may differ in the cement seam overlap, type of sewn seam, heat shrinking procedures, and temperature.

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2-.5 AIRCRAFT FABRIC-NATURAL. .b attachment. Reinforcing tape is used over Physical specifications and minimum strength the rib cab on top of the fabric and for inter-rib requirements for natural fiber fabric, cotton bracing. and linen, used to recover or repair components of an aircraft, are listed in .c Finishing Tape, sometimes referred to as table 2-1. Tear resistance is an important surface tape, should have the same properties factor when considering aircraft fabric. A test as the fabric used to cover the aircraft. method such as ASTM D 1424 is recommended. Technical Standard Order .d Lacing Cord shall have a minimum TSO-C15d, entitled Aircraft Fabric, Grade A breaking strength of 40 lb. Lacing cord (AMS 3806D); and TSO-C14b, Aircraft meeting the specifications listed in table 2-2 is Fabric, Intermediate Grade (AMS 3804C) acceptable. Rib lace cord should have a current edition, respectively, describe the microcrystalline fungicidal wax, paraffin-free minimum standards that all fabric must meet wax, or beeswax coating, or other approved to qualify as aircraft covering material. treatment to prevent wearing and fraying when pulling through the structure. 2-.6 RECOVERING AIRCRAFT. Recover or repair aircraft with a fabric of equal quality .e Machine Thread shall have a minimum and strength to that used by the original breaking strength of 5 lb. Thread meeting the aircraft manufacturer. It is recommended that specifications listed in table 2-2 is acceptable. fabric conforming to TSO-C15d or TSO-C14b be used to recover aircraft originally covered .f Hand-Sewing Thread shall have a with lower strength fabric conforming to minimum breaking strength of 14 lb. Thread AMS 3802, current edition. meeting the specifications listed in table 2-2, is acceptable. When covering with STC- NOTE: Recovering or repairing air- approved fabric covering material, use the type craft with any type fabric and/or coat- of sewing thread approved by the STC and ing other than the type used by the manufactured under the specific PMA. original aircraft manufacturer is considered a major alteration. Obtain .g Flutter Precautions. When re-covering approval from the FAA on fabric and or repairing control surfaces, especially on installation data. Cotton and linen rib high performance airplanes, make sure that lacing cord, machine and hand-sewing dynamic and static balances are not adversely thread, and finishing tapes should not affected. Weight distribution and mass be used with polyester and glass fabric balance must be considered to preclude to covering. possibility of induced flutter.

.a Reinforcing tape minimum tensile 2-.7 PREPARATION OF THE strength is listed in table 2-2. Reinforcing tape STRUCTURE FOR COVERING. One of meeting specification MIL-T-5661, Type I, the most important items when covering current edition, is acceptable. Reinforcing aircraft is the proper preparation of the tape should have a minimum 40 lb. resistance structure. Before covering, the airframe must without failure when static tested in shear be inspected and approved by a FAA-certified against a single rib lace, or a pull-through mechanic or repair station. resistance when tested against a single-wire clip, rivet, screw, or any other type of fabric- to-rib

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TABLE 2-1. Cotton and linen fabrics. Minimum Minimum Minimum Materials Specification Tensile Tearing Tensile Thread Use and Strength New Strength New Strength Count Per Remarks (undoped) (undoped) Deteriorated Inch (ASTM D 1424) (undoped) For use on all Airplane cloth TSO-C15d, as 80 pounds 5 pounds 56 pounds 80 min., aircraft. mercerized amended, per inch warp and fill. per inch. 84 max. Required on cotton references warp and fill. warp and fill. aircraft with (Grade “A”). Society wing loading Automotive of 9 p.s.f. or Engineers greater or AMS 3806d, placard never as amended or exceed speed MIL-C-5646 of 160 m.p.h. or greater. For use on Airplane cloth TSO-C14b, as 65 pounds 4 pounds warp 46 pounds per 80 min., aircraft with mercerized amended, per inch warp and fill. inch. 94 max. wing loading cotton. references and fill. warp and fill. less than Society 9 p.s.f. and Automotive never exceed Engineers speed of less AMS-3804c, than as amended. 160 m.p.h. For use on Airplane cloth Society 50 pounds per 3 pounds warp 35 pounds per 110 max. gliders with mercerized Automotive inch warp and and fill. inch. warp and fill. wing loading cotton. Engineers fill. of 8 p.s.f. or AMS 3802, as less, provided amended. the placarded never-exceed speed is 135 m.p.h. or less. This material Aircraft linen. British 7F1. meets the minimum strength Requirements of TSO-C15.

.a Battery Box Treatment. An asphaltic, .c with a # 44 (0.086) drill bit. Size rubber-based acid-proof coating should be 6 screws, drill bit size # 36 (0.1065), may be applied to the structure in the area of a battery installed in stripped or worn holes drilled for box, by brush, for additional protection from size 4 screws, usually without redrilling. Worn battery acid. Control cables routed in the area holes for wire clips and wire barbs should be of the battery box should be coated with redrilled a minimum 1-1/2 hole distance from paralketone. the original locations using a drill jig to ensure correct spacing, with the appropriate size drill .b Worn Holes. Oversized screw holes or bit. Drill bit size # 30 (0.128) may be used to worn size 4 self-tapping screw holes through redrill oversize holes for 1/8-inch diameter ribs and other structures used to attach fabric blind rivets a minimum 1-1/2 hole diameter may be redrilled a minimum 1-1/2 hole distance from the original location. diameter distance from the original hole location

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TABLE 2-2. Cotton and Linen, Tapes and Threads.

Minimum Yards Per Materials Specification Yarn Size Tensile Strength Pound Use and Remarks Used as reinforcing tape on fabric and Reinforcing MIL-T-566 1 E, 150 pounds per under rib lacing cord. Strength of other tape, cotton. Type 1 1/2 inch width. widths approx. in proportion. MIL-Y-1140H Lacing cord, Lacing fabric to structures. Unless Federal T-C-57 40 pounds. 310 minimum. prewaxed already waxed, must be lightly waxed 1F braided cotton. before using. Lacing cord, Lacing fabric to structures. Unless MIL-C-5648A 80 pounds. 170 minimum. braided cotton. already waxed, must be lightly waxed before using. Lacing cord Lacing fabric to structures. Unless thread, high MIL-T-5660B Ticket No. 62 pounds. 480 minimum. already waxed, must be lightly waxed tenacity cotton. 10. before using. Machine Federal V-T- thread cotton 276H 20/4 ply 5 pounds. 5,000 nominal. Use for all machine sewing. Hand-Sewing Federal V-T- Use for all hand-sewing. thread cotton. 276H Type III B 8/4 ply 14 pounds. 1,650 nominal. Use fully waxed thread. Finishing Use over seams, leading edges, trailing (Surface) tape Same as fabric Same as fabric edges, outer edges and ribs, pinked, cotton. used. used. raveled or straight edges.

.d Fairing Precautions. Aluminum leading .f and stainless steel parts need not be dope- edge replacement fairings installed in short proofed. sections may telescope during normal spar bending loads or from thermal expansion and ()1 A solvent-sensitive primer on contraction. This action may cause a wrinkle ferrous metal and aluminum alloy components to form in the fabric, at the edge of the lap which will be in contact with fabric may be joint. Leading edge fairing sections may be protected from solvent damage by overcoating fastened together with rivets or screws to with a two-part epoxy primer. Epoxy primer prevent telescoping after installation. Trailing meeting MIL-P-53022B is acceptable. edges should be adequately secured to prevent movement and wrinkles. ()2 Small metal or wood surfaces, such as rib caps, to which fabric will not be dope .e Dope Protection. Solvents found in bonded as a part of the particular fabric nitrate and butyrate dope will penetrate, attachment procedure may be protected from wrinkle, lift, or dissolve most one-part wood dope damage by cellophane tape or aluminum varnishes and one-part metal primers. All foil. wood surfaces that come in contact with doped fabric should be treated with a protective .g Chafe Protection. Fabric and finishing coating such as aluminum foil, cellulose tape, tape is often cut through with sandpaper over or dope-proof paint to protect them against the sharp edges during the coating and finishing action of the solvents in the dope. This can procedure and later polishing. All sharp metal also be accomplished by recoating with a edges or protruding screws, nails, rivets, and suitable, solvent resistant two-part epoxy bolt heads should be covered with an anti- varnish, which will be impervious to solvent chafe tape to prevent cutting and wearing penetration and damage after curing. Clad through the fabric after installation. Use aluminum

4 Par 2-6 9/8/98 AC 43.13-1B appropriate non-bleeding cotton adhesive coat- .j glass fiber deck cloth instead of fabric, no ed tape, finishing tape, or strips of fabric, cut sealer or dope should be applied to the from the fabric being used to cover the air- plywood as it would inhibit penetration of craft, doped in place. epoxy resin.

()1 Small holes cut through the fabric to ()1 Sand plywood surfaces as needed to accommodate flying wires, control cables, and remove old loose dope or varnish residue to fittings, must be reinforced with finishing tape provide a clean bonding surface. Remove any or fabric patches cut from the same fabric used oil, grease, or other contamination with a for the covering. suitable solvent such as naphtha. Small, rough areas and irregularities in the plywood surface ()2 Areas needing additional chafe and around any plywood repairs may be filled protection such as control cables routed firm and smoothed with an appropriate commercial against the fabric surface should be protected grade wood filler. Filling large warp with patches cut from cotton duck, leather, or depressions on plywood surfaces with a wood plastic. These patches may be sewn, doped, or filler for cosmetic purposes is not acceptable. cemented in place, as appropriate. ()2 After cleaning and sanding all ()3 Any drag and anti-drag wires in the plywood surfaces, seal the wood grain with a wings should be protected from chafing at suitable solvent resistant two-part epoxy cross points. varnish. After the varnish has thoroughly dried, apply two brush or spray coats of clear .h Inter-Rib Bracing. Use a woven fabric dope, allowing sufficient drying time between tape of the same quality and width as that used coats. for the rib lace reinforcing, where so incorporated in the wing design by the original 2-.8 FABRIC SEAMS. Seams parallel to the aircraft manufacturer. When the original line of flight are preferable; however, spanwise routing for the inter rib bracing is not known, seams are acceptable. the tape will be routed diagonally, alternating between the top and bottom of each rib cap on .a Sewn Seams. each successive rib, if a single pair, half way between the front and rear spars. The number ()1 Machine-sewn seams should be of tape pairs will duplicate the original aircraft double stitched using any of the styles manufacturer’s installation. Tapes will be illustrated in figure 2-1 A, B, C, or D. A routed continuously from the wing butt to the machine-sewn seam used to close an envelope wingtip bow, with one turn of tape around at a wingtip, wing trailing edge, empennage each intermediate rib cap strip. Care should be and control surface trailing edge, and a given to position the tape so as not to interfere fuselage longeron may be made with a single with control cables, bellcranks or push-pull stitch when the seam will be positioned over a rods. structure. (See figure 2-1 E.) The envelope size should accommodate fittings or other .i Preparation of Plywood Surfaces for small protrusions with minimum excess for Covering. Prior to covering plywood installation. Thick or protruding leading edge surfaces, prepare the surface by sanding, sewn seams should be avoided on thin airfoils cleaning, and applying sealer and dope. When with a sharp leading edge radius because they plywood surfaces are to be covered with light may act as a stall strip. weight

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()2 Hand sew, with plain overthrow or ()7 that will minimize any protrusions. baseball stitches at a minimum of four stitches Cover the seams with finishing tape at least per inch, or permanent tacking, to the point 3 inches wide, centering the tape on the seam. where uncut fabric or a machine-sewn seam is reached. Lock hand sewing at a maximum ()8 Sewn seams parallel to the line of of 10 stitch intervals with a double half hitch, flight (chordwise) may be located over ribs. and tie off the end stitch with a double half However, careful attention must be given to hitch. At the point where the hand-sewing or avoid damage to the seam threads by rib lace permanent tacking is necessary, cut the fabric needles, screws, rivets, or wire clips that are so that it can be doubled under a minimum of used to attach the fabric to the rib. Cover 3/8 inch before sewing or permanent tacking is chordwise seams with a finishing tape at least performed. (See figure 2-2.) 3 inches wide with the tape centered on the seam. ()3 After hand sewing is complete, any temporary tacks used to secure the fabric over .b Doped Seams. wood structures may be removed. ()1 For an overlapped and doped ()4 Cover a sewn spanwise seam on a spanwise seam on a wing’s leading edge, wing’s leading edge with a minimum 4-inch overlap the fabric at least 4 inches and cover wide pinked-edged surface tape with the tape with finishing tape at least 4 inches wide, with centered on the seam. the tape centered at the outside edge of the overlap seam. ()5 Cover a spanwise-sewn seam at the wing trailing edge with pinked-edge surface ()2 For an overlapped and doped tape that is at least 3 inches wide. For aircraft spanwise seam at the trailing edge, lap the with never-exceed speeds in excess of fabric at least 3 inches and cover with pinked- 200 mph, cut V notches at least 1 inch in depth edge surface tape at least 4 inches wide, with and 1/4 inch in width in both edges of the the tape centered on the outside edge of the surface tape when used to cover spanwise overlap seam. seams on trailing edges of control surfaces. Space notches at intervals not exceeding ()3 For an overlapped and doped seam 6 inches. On tape less than 3 inches wide, the on wingtips, wing butts, perimeters of wing notches should be 1/3 the tape width. In the control surfaces, perimeters of empennage event the surface tape begins to separate surfaces, and all fuselage areas, overlap the because of poor adhesion or other causes, the fabric 2 inches and cover with a finishing tape tape will tear at a notched section, thus that is at least 3 inches wide, centered on the preventing progressive loosening of the entire outside edge of the overlap seam. length of the tape which could seriously affect the controllability of the aircraft. A loose tape ()4 For an overlapped and doped seam acts as a trim tab only on a movable surface. It on a wing’s leading edge, on aircraft with a becomes a spoiler on a fixed surface and has velocity never exceed (Vne) speed up to and no effect at the trailing edge other than drag. including 150 mph, overlap the fabric 2 inches and cover with a finishing tape that is at least ()6 Make spanwise-sewn seams on the 3 inches wide, with the tape centered on the wing’s upper or lower surfaces in a manner outside edge of the overlap seam.

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FIGURE 2-1. Fabric seams.

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TEMPORARY TACKS MAY BE USED TO SECURE FABRIC IN POSITION FOR HAND- SEWN SEAMS OVER A WOOD STRUCTURE. PERMANENT TACKS MAY BE USED TO SECURE FABRIC AT WING BUTT STRUCTURES IN LIEU OF A HAND-SEWN SEAM AND SHOULD BE OF THE APPROPRIATE LENGTH AND GAUGE, AND MADE OF CORROSION RESISTANT STEEL OR BRASS.

FIGURE 2-2. Typical methods of attaching fabric.

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()5 For an overlapped and doped seam ()2 An alternative method, when fabric on the perimeter of a wing (except a leading of sufficient width is available, is to sew edge), perimeters of wing control surfaces, together, side-by-side, two sections of fabric, perimeters of empennage surfaces, and all placing the seams spanwise on the leading areas of a fuselage, on aircraft with a Vne edge, then fit and sew the wingtip and trailing speed up to and including 150 mph, overlap edge in the same manner as the multiple piece the fabric 1 inch and cover with a finishing chordwise envelope. tape that is at least 3 inches wide, centered on the outside edge of the overlap seam. ()3 An envelope may be developed for the fuselage in the same manner, with a final 2-.9 COVERING METHODS. The method closing along a longeron by hand-sewn or of fabric attachment should be identical, as far overlapped and doped seams in accordance as strength and reliability are concerned, to the with the aircraft Vne speed. method used by the manufacturer of the airplane being recovered or repaired. .b The Blanket Method. A blanket is Carefully remove the old fabric from the developed by sewing together, side-by-side, airframe, noting the location of inspection multiple sections of fabric with the seams covers, drain grommets, and method of chordwise or two wide sections of fabric, attachment. Cotton or linen fabric may be side-by-side, placing the seam spanwise on the applied so that either the warp or fill-threads leading edge, the same as an envelope. Close are parallel to the line of flight. Either the the three remaining sides with a hand-sewn envelope method or blanket method of seam or overlapped and doped seams in covering is acceptable. accordance with the aircraft Vne speed. Small components may be covered by wrapping one .a The Envelope Method. A wing piece of fabric over a straight leading or envelope may be developed by two methods. trailing edge, then closing three sides with Machine sew together, side by side multiple hand-stitched or overlapped and doped seams fabric sections, cut to reach chordwise around in accordance with the aircraft Vne speed. the wing, starting and ending at the trailing edge with a minimum of 1 inch excess length. NOTE: All overlapped and doped The sewn envelope is then positioned around seams will be made only over underly- the wing and secured with closely spaced ing supporting structures extending T-Head pins at the wingtip and trailing edge. the full width of the seam. Excess material may then be trimmed. Carefully remove the envelope and complete .c Machine-sewn alternate. An alternate by machine sewing at the wingtip and along to machine-sewn seams on a wing envelope or the trailing edge, except where the geometry of blanket is to use two sections of wide fabric the wing (aileron and flap cut out) would spanwise. Attach the fabric with overlapped prevent the sewn envelope from being and doped seams at the leading and trailing reinstalled. After reinstalling the envelope, the edge, wingtip and wing butt, in accordance un-sewn sections and butt end are then closed with the aircraft Vne speeds. (Reference by hand-sewn or overlapped and doped seams paragraph 2-7 b.) Smaller components may be in accordance with the aircraft Vne speed. covered in the same manner. The fuselage may (Refer to paragraph 2-7 b.) be covered with multiple fabric sections with

Par 2-7 Page 2-9 AC 43.13-1B 9/8/98 overlapped and doped seams on the longerons .c Reinforcing tapes should be of the or other wide fabric-forming structures in ac- appropriate width for hardware attachment cordance with the aircraft Vne speed. (Refer- such as screws, rivets, wire clips, etc., which ence paragraph 2-7 b.) pierce the center of the tape. Reinforcing tape under cord lacing should be the same width as .d Holes in Fabric. Never cut any holes in the rib to which the fabric is laced and may be the fabric for inspection panels, spar fittings, comprised of multiple widths positioned or drain grommets; or attach the fabric to the side-by-side to achieve the required width. airframe with rib lacing screws, rivets, clips, or rib stitch cord until the fabric has been semi- .d When the aircraft Vne speed is over tauted and stabilized with several coats of 250 mph, anti-tear strips, cut from the same dope. quality fabric used to cover the aircraft, are recommended for use under reinforcing tape 2-.10 REINFORCING TAPE. on the entire top surface of the wing and on the portion of the wing’s bottom surface in the .a Reinforcing tape should be securely propeller slipstream. The propeller slipstream bonded to the fabric surface with dope before is considered to be the propeller diameter plus cord lacing or installation of hardware. Where one outboard rib. The anti-tear strip should be multiple attachments are in close proximity, installed completely around the wing, such as on a wing rib, continuous reinforcing beginning and ending at the trailing edge in the tape should be installed, extending at least propeller slipstream, and installed from the 1 inch past the last attachment at each end. trailing edge over the leading edge and back Random or wide spacing, such as on fuselage under to the front spar on the balance of the stringers or empennage surfaces, may be ribs. Anti-tear strips should extend 1/2 inch reinforced with 2-inch lengths of reinforcing past the wing rib cap edges and be thoroughly tape centered on the attachment location. bonded to the fabric with dope before the reinforcing tape is installed. (See figure 2-3.) .b

FIGURE 2-3. Exploded side view of rib.

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2-.11 LACING. interference with the routing of any control cable, bellcrank, or any other movable item. .a Fabric should be attached to aircraft To prevent chafing and cutting of the lacing components to prevent ballooning due to cord, control cables or any other movable aerodynamic forces, in the identical manner items should be tensioned or positioned to and locations as used by the original aircraft their normal alignment before rib lacing and manufacturer. Any deviation from the original checked afterwards to ensure adequate method(s) of attachment, such as screws, clearance. When a lace cord will be chafed by a rivets, wire clips, lacing cord, etc., are moving component, a blindstitch may be made considered a major alteration and in conflict around the top and bottom rib caps as with the aircraft type design data. Obtain FAA illustrated in figure 2-11. approval on any deviation. .c Stationary structures interfering with NOTE: When the type of rib lace needle routing may be circumvented by knot used by the original aircraft aligning the needle forward or aft adjacent to manufacturer is not known, the modi- the rib cap. Pull the needle through the wing fied seine knot shown in figure 2-4 and then return through the same hole and exit through figure 2-9c will be used. at the desired adjacent location.

NOTE: The first lace on a wing rib should be spaced from the leading edge fairing no more than 1/2 the required lace spacing for the balance of the rib.

.d Both surfaces of fabric covering on wings and control surfaces must be securely fastened to the ribs by lacing cord or any other method originally approved for the aircraft. Care must be taken to insure that all sharp edges against which the lacing cord may bear are protected by tape in order to prevent abrasion of the cord. Separate lengths of lacing cord may be joined by the splice knot shown in figure 2-10 or tied off. The first loop is tied with a square knot as illustrated in figure 2-5 and figure 2-9a, and the knot secured with a half hitch on each side after the lacing is pulled tight around the rib. The needle is then routed through the wing and around the rib cap at the

FIGURE 2-4. Standard external modified seine knot used for single and double rib lacing.

.b During the installation of lacing cord through a wing or any other component, special attention should be given to avoid

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FIGURE 2-5. Starting stitch for rib lacing.

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FIGURE 2-6. Standard single-loop lacing.

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FIGURE 2-7. Standard knot for double-loop lacing.

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FIGURE 2-8. Standard double-loop lacing (optional).

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FIGURE 2-9a. Alternate sequence to tie a modified seine knot for rib lacing.

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FIGURE 2-9b. Alternate sequence to tie a modified seine knot for rib lacing.

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Step 9. Pull cord section “D” perpendicular to the fabric Step 10. After all slack is removed by pulling cord section “D,” surface to remove all slack in the cord back to the last switch hands and place a thumbnail on the loose rib lacing knot while working the loose knot to the knot formed on the right-hand side, then secure the right side. Do not pull cord section “E.” knot by pulling firmly perpendicular to the fabric surface on cord section “E.”

Step 11. After completing all lacing in the same sequence, the end is secured with a half hitch after the modified seine knot. The knot is pulled to the inside by routing the needle thru the wing before cutting the cord to leave the end inside.

FIGURE 2-9c. Alternate sequence to tie a modified seine knot for rib lacing.

18 Par 2-10 9/8/98 AC 43.13-1B next rib lace location with the cord and knot .f aircraft manufacturer, should be remaining on top of the fabric surface as illus- reinstalled in the same location and spacing as trated in figure 2-5, figure 2-6, and figure 2-8. installed by the original aircraft manufacturer. An alternate method is to route the needle un- The lace cord is routed around the stringer, rib der the fabric and out through the next lace lo- cap, or other structure using an appropriate cation, then back down through the wing as il- length, single or double pointed, curved needle lustrated in figure 2-9a through figure 2-9c. A as illustrated in figure 2-11. Blind laces are modified seine knot is then tied as illustrated tied with a square knot, then pulled tight and in figure 2-4 through figure 2-9c. secured with a half hitch at each side. The lace may be rotated to place the knot under the ()1 Rotate each lace loop to place the fabric surface before cutting the cord. knot at the side of the rib cap to reduce the protrusion and aerodynamic interference before moving to the next lace location, or the cord routed under the fabric to the next lace location as illustrated in figure 2-9a through figure 2-9c. The end cord is then cut off leaving a minimum of 1/4 inch stub. Lacing tension should be uniform.

()2 Repeated pulling of long lengths of lacing cord may remove wax coating from the cord and cause fraying. Convenient lengths of rib lacing cord may be used to lace long or thick ribs. The end of each length is tied off The splice knot is made by crossing the ends of the cord, and making four complete with a half hitch as illustrated in figure 2-9c, or wraps with the small end of the free piece around if needed, separate lengths of lacing cord may the end of the standing piece. The end is then be joined by using the splice knot illustrated in doubled back through the formed loop. The other free end is wrapped and doubled back. The long figure 2-10. ends of the cords are then pulled until the knot is tight. The short ends are cut close to the knot. ()3 Lacing is installed through other This finishes the splice knot. components, where applicable, in the same manner as a wing. Single, wide space lace FIGURE 2-10. Splice knot. attachments, usually used on empennage surfaces, are tied with a square knot and half 2-.12 STITCH SPACING. hitch on each side, the same as a starting wing rib lace illustrated in figure 2-9a, steps 1 and 2. .a Rib lace spacing on wings, formerly The lace may be rotated to place the knot referred to as stitch spacing, should be no under the fabric before cutting the cord. greater than the spacing used by the original aircraft manufacturer. When the original .e Blind lacing on a fuselage, wing rib caps spacing cannot be determined the maximum above and below a fuel tank, and any other spacing illustrated in figure 2-12 should be component, when used by the original used on the wings and wing control surfaces.

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FIGURE 2-11. Blindstitch lacing - square knot secured with half hitches.

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FIGURE 2-12. Fabric attachment spacing

.b When the original lace spacing on the empennage surfaces and fuselage, is not known, a maximum spacing of two times the spacing shown in figure 2-12 for the slipstream area (prop wash) on the wings may be used.

.c The installations of fabric attachments such as screws, rivets, wire clips, and rib lacing should be delayed until the fabric is stabilized and pulled taut with dope. This action is delayed to avoid pulling wing ribs and other structures out of alignment or tearing the fabric at attachment points as the fabric becomes taut. All lacing should be installed adjacent to the structure to which the fabric is being laced, to avoid tearing the fabric and/or creating slack in the cord loop when a load is applied. Where plastic washers were used by the aircraft manufacturer to provide increased pull-through resistance, under the heads of rivets or screws, the same diameter aluminum washer may be used as replacement. Aluminum washers are used because they are not

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2-.13 FASTENERS. Several light aircraft designs employ screws, rivets, or single-wire metal clips to secure the fabric to the wing.

.a Screws holding the old fabric can be removed after spinning a small sharpened tube around each screw or using a razor blade to cut and peel away the finishing tape. Care must be taken not to mark or scribe the underlying metal or wood structure. Blind rivets through ribs can be removed by drilling in the center to undercut the head.

.b Single-wire clips may be removed without damage to the rib by inserting a wide, thin screwdriver blade under the clip and carefully twisting. Apply a lifting force at the clip end to pull it up through the hole.

22 Par 2-11 9/27/01 AC 43.13-1B CHG 1

NOTE: It is important that any dam- .c width should be sufficient to bond the age found to ribs, such as oversize riv- fabric a minimum of 3/8 inch on each side of et or screw holes, and cracks or breaks all fabric attachments. Two inch width tape is in the rib cap, should be tagged imme- normally used. Tapes over wing rib lacing diately for easy location and repair should extend a minimum of 1/2 inch past later. each end of any reinforcing tapes. Random or widely-spaced attachments may be covered by .c When repairs are made to fabric individual sections of fabric or finishing tape. surfaces attached by special mechanical methods, duplicate the original type of .d Installation of finishing tapes for fastener. When self-tapping screws are used additional wear resistance is recommended for the attachment of fabric to the rib structure, over the edges of all fabric-forming structures. observe the following procedure: This includes fuselage stringers, longerons, leading and trailing edges, false or nose ribs, ()1 Redrill the holes where necessary control surfaces, and empennage ribs not due to wear, distortion, etc., and in such cases, already covered and protected by a finishing use a screw one size larger as a replacement. tape that is required to be on a fabric seam or fabric attached to the structure. Compound ()2 Extend the length of the screw surfaces, such as wingtip bow and empennage beyond the rib capstrip at least two threads. surfaces, are more conveniently taped using bias cut finishing tape, which easily conforms ()3 Install a thin washer, preferably to the compound contour, rather than notching aluminum, under the heads of screws and dope linear cut tape to fit the surface. Bias cut tape pinked-edge tape over each screw head. will be reduced to approximately two thirds the original cut width when pulled tight around 2-.14 FINISHING TAPE. a wingtip bow and should be considered when selecting the width of tape for the various .a Finishing tape (surface tape) is installed locations. after the fabric has been pulled taut with the initial dope application. This procedure is .e Finishing tapes are applied by coating performed to prevent ripples from forming in the fabric surface over which the tape will be fabric panels adjacent to newly applied tapes. applied with dope, applying the tape over the Ripple formation is caused by the inability of wet dope film, then brushing the tape firmly the combined tape and fabric to tighten onto the fabric surface. This action will assure uniformly with adjacent fabric when additional a good bond by thoroughly saturating and dope is applied. wetting the finishing tape.

.b In addition to the tape widths required 2-.15 INSPECTION RINGS AND DRAIN to be installed over fabric seams specified in GROMMETS. paragraph 2-7, finishing tape should be installed as weather protection over all rib .a Inspection Rings. Inspection access is lacing, screws, rivets, wire clips, or other provided adjacent to or over every control devices used to secure fabric. This includes bellcrank, drag-wire junction, cable guide, wings, control surface ribs, empennage surface pulley, wing fitting, or any other component ribs, and fuselage stringers, where so installed throughout the aircraft which will be inspected by the original aircraft manufacturer. Tape or serviced annually. They are installed only

Par 2-11 Page 2-23 on the bottom side of the wings except where .c through openings in the sides and top, installed on the top surface by the original and when flying, everywhere throughout the manufacturer. structure. Taxiing on wet runways also splashes water up through any bottom holes. ()1 Cutting the holes may be delayed Therefore, provisions must be made to drain until needed; however, all covers should be water from the lowest point in each fabric finished in matching colors with any trim lines panel or plywood component throughout the and stored until needed. Spraying matching airframe while in a stored attitude. Drain colors a year later is expensive and time holes also provide needed ventilation. consuming. ()1 Install drain grommets on the under ()2 The 3-9/16 inch inside diameter side of all components, at the lowest point in cellulose acetate butyrate (CAB) plastic each fabric panel, when the aircraft is in stored inspection access rings have become popular attitude. Seaplane grommets, which feature a and bond satisfactorily with Nitrate Dope or protruding lip to prevent water splashes Fabric Cement. Any metal inspection hole through the drain hole, are recommended over reinforcements of a particular shape or special drain holes subject to water splashing on land design or size, installed by the original planes as well as seaplanes. The appropriate- manufacturer, should be reinstalled after size holes must be cut through the fabric cleaning. before installing seaplane grommets. Plastic drain grommets may be doped directly to the ()3 Tapes or patches over aluminum fabric surface or mounted on fabric patches reinforcements are optional, but recommended then doped to the covering. Installing a small in the prop-wash areas on the wings and fabric patch over flat grommets to ensure forward fuselage bottom. security is optional. Alternate brass grommets are mounted on fabric patches, then doped to ()4 Fabric patches over plastic rings are the fabric. strongly recommended because plastic is not a stable material, becomes brittle at low ()2 After all coating applications and temperatures, and fatigues and cracks from sanding are completed, open all holes through prop blast vibration. Plastic rings are often flat drain grommets by cutting through the cracked during removal and installation of fabric with a small-blade knife. Do not spring, clip-held covers. Patches with a attempt to open drain holes by punching with a minimum 1-inch overlap, should be installed sharp object because the drain hole will not with dope. remain open.

.b Drain Grommets. Atmospheric 2-15.2-19. [RESERVED.] temperature changes cause the humidity in the air to condense on the inside of aircraft surfaces and pool in all low areas. Rainwater enters